For vigorous growth, plants require primary nutrients, such as carbon, hydrogen, oxygen, nitrogen, phosphorus and potassium, as well as secondary nutrients and micronutrients. Calcium, sulfur and magnesium are considered secondary nutrients and are generally required in smaller quantities than primary nutrients. Micronutrients are required in very small quantities, and include zinc, manganese, iron, copper, molybdenum, boron, chlorine, cobalt and sodium.
Available potassium in the soil is typically augmented with potash (also known as muriate of potash (MOP) or potassium chloride). Available phosphorus in the soil is frequently augmented with solid phosphate fertilizer, typically as monoammonium phosphate (MAP), diammonium phosphate (DAP), or calcium dihydrogen phosphate or monocalcium phosphate (Ca(H2PO4)2), known as triple superphosphate (TSP). Available nitrogen may be augmented by urea. These primary nutrient fertilizers are typically known as NPK fertilizers
MAP and DAP production methods are well-established and conventionally include the reaction of phosphoric acid (H3PO4) with ammonia (NH3) in an exothermic reaction.H3PO4+NH3=>(NH4)H2PO4(MAP)H3PO4+2NH3=>(NH4)2HPO4(DAP)The reactions may take place in a preneutralizer or a pipe cross reactor (PCR), A preneutralizer is a stirred reactor that produces a slurry of ammonium phosphate. A pipe cross reactor is a pipe-shaped reactor where ammonium phosphate is formed by reacting ammonia and phosphoric acid.
In either case, MAP or DAP or a combination of the two may be produced, depending on the ratio of the ammonia and phosphoric acid reactants. The ammonium phosphate slurry produced in the preneutralizer is supplied to a granulator. Ammonium phosphate formed in a PCR is sprayed into a granulator because the ammonium phosphate produced in the PCR is molten.
Calcium dihydrogen phosphate or monocalcium phosphate (Ca(H2PO4)2), known as triple superphosphate (TSP), may be produced by reacting phosphoric acid with phosphate rock.
Generally, solid fertilizer particles are formed by granulation, pelletization or compaction. A granulator is a device for forming granules of fertilizer product. Commonly used granulators are well known in the art and include spray dry granulators, drum granulators, paddle mixers (pug mills) or pan granulators. Preferably, the mixture is pumped and distributed on a rolling bed of material in a drum granulator. Water and/or steam can be fed to the granulator to control the temperature of the granulation process. Granules are then dried and screened, with oversize granules and undersized material (so-called off-spec fines) are recycled back to the granulator. The oversize material may be crushed or ground first before being fed back into the granulator. The undersized and crushed oversized material which is recycled serves a valuable purpose in that it provides seed particles to spur granule formation in the granulator.
In contrast, pelletization is a process by which a powder material is formed into a pellet by compression. As a first step, the fertilizer material may be treated, such as in a crusher, hammer mill or a similar apparatus, to produce a powder comprising relatively uniform small particles, typically less than about 0.70 mm or fine enough to pass through a standard US 25 mesh screen. Secondary and micronutrients in powder form may be incorporated and mixed with the powder as additives at this stage. The powder material may then be mixed and wetted with a small amount of water in preparation for pelletization. Once the mixture has been adjusted to a suitable moisture content, it may be pelletized using a pellet former, such as a mill or press, which uses compression to produce pellets. Suitable pellet mills are well known in the art and may include screw-type extrusion pellet mills.
Compaction is similar to pelletization in that a compressive force is used to produce a pellet, but differs from pelletization in that it uses significantly greater compressive force to cohere the solid material together. Good quality compacted product requires appropriately sized raw materials in powdered form. Since the raw materials are usually available only in a coarse form, they have to be put through a grinder to obtain a fine powder.
However, any grinding or compaction process which processes sulphur has a serious inherent risk of fire or explosion. Elemental sulphur is flammable, and could potentially cause fire or explosion during the production process. The grinding media is bound to generate heat which could ignite the flammable raw material causing explosion.